1. Field of the Invention
The present invention relates to an Orthogonal Frequency Division Multiplexing (OFDM) cellular system, and more particularly, to a method of searching for a cell in an OFDM cellular system, an apparatus using the method, a system using the method, and a frame structure used in the method.
2. Description of the Related Art
In a Wideband Code Division Multiple Access (WCDMA) method of the 3rd Generation Partnership Project (3GPP), a cellular system uses a total of 512 long Pseudo-Noise (PN) scrambling codes in order to identify base stations of a forward link, wherein each adjacent base station uses a unique long PN scrambling code as a scrambling code of forward link channels. When a mobile station is turned on, the mobile station must acquire system timing of a base station to which the mobile station belongs (i.e., a base station of which a reception signal has the maximum amplitude) and a long PN scrambling code ID used by the base station. This process is called a mobile station's cell search process.
In WCDMA, in order to easily perform the mobile station's cell search process, the 512 long PN scrambling codes are grouped into 64 groups, and a primary sync channel and a secondary sync channel are included in the forward link. The primary sync channel is used for a mobile station to acquire slot sync, and the secondary sync channel is used for the mobile station to acquire a 10-msec frame boundary and long PN scrambling code group ID information.
The mobile station's cell search process in WCDMA is accomplished in 3 steps. In the first step, a mobile station acquires slot sync using a Primary Scrambling Code (PSC). In WCDMA, the same 15-slot PSC is transmitted every 10 msec, and PSCs transmitted by all base stations are the same signal. In the first step, slot sync is acquired using a matching filter suitable for the PSC.
In the second step, long PN scrambling code group ID information and a 10-msec frame boundary are acquired using the slot timing information acquired in the first step and a Secondary Scrambling Code (SSC).
In the third step, a long PN scrambling code ID used by a currently connected base station (BS) is acquired using the 10-msec frame boundary and the long PN scrambling code group ID information that were acquired in the second step and a common pilot channel code correlator. That is, since 8 long PN scrambling codes are mapped to a single code group, the mobile station detects the long PN scrambling code ID used in the current cell on the basis of outputs of the common pilot channel code correlator for 8 long PN scrambling codes corresponding to the long PN scrambling code group ID information.
In WCDMA, a sync channel consists of a primary sync channel and a secondary sync channel, and the primary sync channel, the secondary sync channel, a common pilot channel, and other data channels are multiplexed in a CDMA method based on a time domain direct sequence spread spectrum.
Recently, in the 3GPP, an OFDM-based wireless transmission technology standardization is being established as a part of 3rd Generation Long Term Evolution (3G-LTE) to compensate for disadvantages of WCDMA. The sync channel & common pilot channel structure and the mobile station's cell search process used in WCDMA are suitable for Direct Sequence Code Division Multiple Access (DS-CDMA) but cannot be applied to an OFDM forward link. Thus, a forward link sync channel & common pilot channel structure, a mobile station's initial cell search method, and an adjacent cell search method for handover are required in an OFDM cellular system.